For Yam bits try DA marine in Newport-on-Tay.
Stilff tilt is going to be a case of removing the big toob that it all pivots round, and de- gunking it of all the dried grease that will be now well on its way to acting more like a treacle- araldite mix than grease. Then re-grease with fresh - reassemble - job done!
Unless it's one of these ones where the "lock down" is a lever on the top of a non powered hydraulic ram rather than a load of mechanical levers & catches? - there may be other possibilities in that case!